The present invention relates generally to a two-stage, fixed-bed coal gasifier having dual gas off-takes for providing a middle or side stream of relatively tar-free, high temperature gas, and more particularly to such a gasifier wherein movable skirt means are incorporated within the gasifier for selectively positioning the off-take point for the middle gas stream.
Two-stage, fixed-bed gasifiers having dual off-takes for the gases produced during the gasification of coal are well known. An upper stage of the two stages of gasification defines a coal distillation and devolatization zone while a lower stage defines a combustion or gasification zone. A portion of the gaseous products produced in the gasification zone is commonly referred to as a side stream or middle stream and is removed before any contact with the fresh coal so as to be relatively clean and tar free. This middle stream of gas is normally at a temperature of about 1000.degree.-1600.degree. F. Another portion of the hot gases produced in the gasification zone is referred to as the top gas and is directed upwardly through the devolatization zone where it is utilized to gradually increase the temperature of the incoming coal to a level sufficient to liberate and entrain tar and other volatile matter from the coal prior to the discharge thereof through a top gas off-take in the gasifier. This top gas stream exiting the devolatization zone is usually at a temperature in the range of about 250.degree. to about 800.degree. F.
While the success of the fixed-bed, two-stage dual off-take gasifiers has been satisfactory, there are some problems or shortcomings which detract from these gasifiers in their present form. For example the composition and to some extent the temperature of the middle stream of gas is highly dependent upon the point of withdrawal of this middle stream from the gasification zone. The fixed-bed gasifiers as presently known provide the gasifier with a fixed vertically extending annulus having an opening near the gasification zone for splitting the gas prior to or after the exit thereof from the gasification zone into the middle gas stream and the top gas stream for the purpose of providing the middle gas stream with a desired composition. A withdrawal of the middle gas stream from a point near the top of the gasification zone would normally provide for a middle stream of gas with less tar content than would be attainable by withdrawing the middle stream from a point above the gasification zone. However, the composition and temperature of the middle gas stream taken off at a given point in or near the gasification zone is largely dependant upon variables in the process parameters and the reactivity of the coal utilized. For example, coals useable in fixed-bed gasifiers can have significantly different compositions and heating values. Also, the use of differing steam to air ratios and/or different gasifier operating pressures ranging from about atmospheric to about 2000 psig have a considerable effect upon the composition and the temperature of the gases at any given vertical position within a two-stage gasifier. Thus, by merely changing the coal heating values or any of the various operating parameters, any optimization of the composition of the middle gas stream in a gasifier with an annulus in a fixed vertical position becomes very difficult to achieve. Some control over the composition of the middle gas stream may be achieved to some degree of satisfaction by using complex controls over the process parameters for controlling the gasification reactions. One such effort used in the prior art relates to controlling the volume of the gas passing through the devolatization zone to regulate the cracking and other reactions occurring in the devolatization zone and is provided by employing a valve in the middle gas outlet line so as to control the volume of middle gas flowing upwardly through the devolatization zone. While such a valve arrangement does provide for some control over the volume of gases exiting from the gasifier through the middle gas stream off-take, this arrangement can not adequately control the composition of the middle gas stream especially when using coals of different reactivities and different steam to air ratios.